In recent years, with progress of digital technology and improvement in data compression technology, information recording media for recording computer programs, music information, video information (contents), etc., which include optical disks, such as CD (compact disk) and DVD (digital versatile disk), have come to attract attention. And inexpensive optical disk devices for performing recording of information to the optical disk and reproducing of information from the optical disk have come to spread. CD-RW (CD-rewritable), DVD-RAM, DVD-RW (DVD-rewritable), DVD+RW (DVD+rewritable), etc are currently marketed as rewritable optical disks.
In an optical disk device, a small spot of a laser beam is formed on a recording layer of an optical disk in which the track is formed in a spiral shape or in the shape of a concentric circle, to recording information to the disk, and reproduction of information from the disk is performed based on the reflected light from the recording layer.
Meanwhile, the amount of information of video information or contents tends to increase year by year, and it is expected that the amount of information that is recordable on a single optical disk, i.e., the storage capacity, will further increase. For example, the Blu-ray standard is proposed as a method for increasing the storage capacity of an optical disk such that the recording density thereof is higher than that of the currently marketed DVD.
In the case of the optical disk in conformity with the Blu-ray standard (which will be called the BD disk), the thickness of the substrate is equal to about 0.1 mm, and the optical disk device corresponding to the BD disk uses a light source whose wavelength is equal to 405 nm, forms a focusing light spot with NA of 0.85 by using an objective lens, and performs recording, reproduction and erasing of information for the BD disk. That is, the DVD and the BD disk have mutually different light source wavelengths, substrate thicknesses, and NA values.
For this reason, there is a problem in that, in the optical disk device adapted to access both the DVD and the BD disk and having a single objective lens, if the characteristics of the objective lens are made to suit one of the DVD and the BD disk, the aberration will occur for the other recording medium due to the difference in the substrate thickness between the DVD and the BD disk.
Another method for increasing the storage capacity of an optical disk is to use an optical disk having a plurality of recording layers (called the multi-layered disk). Development of an optical element corresponding to the multi-layered disk and an optical disk device adapted to access the multi-layered disk is performed energetically. For example, refer to Japanese Laid-Open Patent Application No. 63-113947, Japanese Patent No. 2988732, and the literature entitled “Volume holographic device for the spherical aberration correction and the parallel data access in three-dimensional memory” Fr-PD-15 ISOM2000, Lee S. C. and Y. Kawata.
However, according to the optical element disclosed in Japanese Laid-Open Patent Application No. 63-113947, to make the number of the recording layers of the optical disk into several ten layers, it is necessary to create the optical disk by using a different recording material for each recording layer, which will make the cost for creating the optical disk too high.
According to the optical disk device disclosed in Japanese Patent No. 2988732, if a plurality of light sources are arranged on the same plane and the light beam from the light source located out of the optical axis is used, the aberration (coma aberration) occurs in the lens system. There is a problem in that it is difficult to obtain the light spot of a diffraction limit on the disk surface. Moreover, the light from the collimating lens is outputted with a certain angle, the quantity of light captured into the objective lens is different for each light source, and there is a problem in that the efficiency for light utilization is reduced significantly.
According to the optical element disclosed in the above-mentioned literature, diffraction efficiency of a volume hologram is reduced as the volume hologram performs multiplex exposure. If the number of spots is increased, the resulting surface power will fall. Moreover, if the light source is arranged on the surface perpendicular to the optical axis, the light from the collimating lens is outputted with a certain angle, and the quantity of light captured into the objective lens and the light quantity distribution are different for every light source. There is a problem in that the reduction of efficiency for light utilization and the asymmetrical spot configuration will occur.